The precision matte focusing screens, although dark with this lens, REALLY help with focusing.

It was my understanding that the benefit of the precision matte focusing screen is really for lenses faster than f/2.8, since the laser etching on the standard screen which is intended to brighten slow (e.g. consumer) lenses limits faster lenses to VF brightness equivalent to about f/2.5. I've certainly confirmed that with my 85L and a standard screen, where stopping down from f/1.2 onwards with DoF preview, the VF does not get any dimmer until I hit ~f/2.5 (although on my 7D, it's clear that the light is getting in since the transmissive LCD in the VF is much more 'washed out' at f/1.2 than f/2.5, even though the scene brightness is unchanged).

Since the 'brightness' of the VF changes in tandem with the DoF seen through the VF, the standard focusing screen is showing you a DoF equivalent to ~f/2.5, even with a wider aperture lens. Thus, precision focusing screens are better for fast lenses because they are showing you the true DoF at apertures faster than f/2.8.

However, I was not aware that there was a significant benefit to the precision matte focusing screens with for manual focusing with f/2.8 (like the MP-E 65mm) and slower lenses. I could see the benefit of a split screen and microprism, but not sure about the benefit from the plain matte screen. Can you elaborate?

I always found the ratings the DSLR screens to be rather optimistic in terms of what speed lens you can manually focus well with. The stock screens, IMO, make it tricky to really nail things with not just stuff that is f/1.4 or f/2 but also f/2.8 and even for f/4 I find they help tremendously (even if things get darker). The stock DSLR screens are so poor for focusing, I almost feel like you don't want to trust them unless you are like f/5.6! They make things across the frame look waymore in focus than the actual photos.

I always found the ratings the DSLR screens to be rather optimistic in terms of what speed lens you can manually focus well with. The stock screens, IMO, make it tricky to really nail things with not just stuff that is f/1.4 or f/2 but also f/2.8 and even for f/4 I find they help tremendously (even if things get darker). The stock DSLR screens are so poor for focusing, I almost feel like you don't want to trust them unless you are like f/5.6! They make things across the frame look waymore in focus than the actual photos.

Thanks for the feedback! Usually with the MP-E 65mm, I'm shooting in Live View, which obviates the focusing screen. But sometimes I use the VF, and I have the Eg-S screen for my 5DII - so, I'll have to give it a try for macro with my f/2.8 lenses (MP-E 65mm and 100mm f/2.8L Macro IS), and maybe with something like the 70-200mm f/2.8L IS II, although AF usually does fine with that lens.

For me MP-E 65 is the reason alone to stick with Canon, and I don't even consider myself macro photographer, more like curious photographer or something.Would like to see people really pushing the envelope with this lens, not just a shots like with let's say 100mm macro with an extension tube.